The present invention relates to an automatic retransmission system transceiver and method which provides the benefit of a muted receiver on both channels. A normal configuration for such systems is a HF radio communication link for medium and long range transmission connected through the retransmission unit to a short range UHF or VHF communication channel. Retransmission of the signal occurs when reception of a properly coded signal is detected by the retransmission unit.
A prior art system such as illustrated by the Moore U.S. Pat. No. 3,702,440 dated Nov. 7, 1972 provides a modulated calling signal for transmission along with an information signal, the amplitude modulated, single frequency or tone, calling signal being utilized to enable a selected receiver for reproducing the received information signal. In systems such as that described by Moore, the calling signal is continuous, the frequency of the calling signal is in the lower part of the audio frequency range, and the frequency of the square wave modulation signal is no greater than about twenty-five percent of the frequency of the tone in the calling signal. There is no amplitude ajustment of the calling signal as a function of the amplitude of the information signal nor is there any time delay to provide a tolerance for incorrect modulating signal detection.
Other prior art systems such as that disclosed in the Cannalte et al U.S. Pat. No. 3,546,574 dated Dec. 8, 1970 utilize signal bursts of a predetermined frequency and duration as the calling signal.
Still other systems such as that disclosed in the Groeneveld et al U.S. Pat. No. 3,084,328 transmit a pilot tone signal having a frequency greater than the audio frequency band. The pilot tone is amplitude modulated by a function of the information channel content.
Yet another system has been by the assignee hereof which detects both the presence and modulation of a pilot tone at a frequency within the channel bandwidth but above the intellegence. In such systems, it has been found that the standards for comparing the detected subaudible rate in the presence of interference often prevent retransmissions.
It is an object of the present invention to provide a nvoel transceiver and method of retransmission of intelligence over communication links of different frequencies.
It is another object of the present invention to provide a novel system and method which prevents the detection of modulating signal harmonics and which eliminates dead zones in the detection band.
It is another object of the present invention to provide a novel transceiver and method in which the control or calling signal is a tone which is modulated in phase at a subaudible rate to prevent the detection of harmonics of the desired subaudible rate in the received information signal as well as to eliminate dead zones within the detection band.
It is yet another object of the present invention to provide a novel transceiver and method in which the standards for the subaudible rate evaluation are varied to provide proper operation although the modulation be disturbed after the second communication link is established.
These and other objects and advantages of the present invention will be apparent from the claims and from the following detailed description when read in conjunction with the appended drawings.